CCLM-based intra-prediction method and device

ABSTRACT

A method for decoding a picture based on a cross-component linear model (CCLM) mode includes deriving neighboring luma reference samples of a luma block, deriving down-sampled neighboring luma reference samples, deriving a linear model parameter based on the down-sampled neighboring luma reference samples and the neighboring chroma reference samples, where the neighboring luma reference samples includes top neighboring luma reference samples, and left neighboring luma reference samples, and where when the top boundary of the luma block overlaps with a boundary of a coding tree unit (CTU), the number of the top neighboring luma reference samples used for deriving the down-sampled neighboring luma reference samples among the neighboring luma reference samples is less than that of the left neighboring luma reference samples used for deriving the down-sampled neighboring luma reference samples.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. § 119(e), this application is a continuation ofInternational Application PCT/KR2019/007582, with an internationalfiling date of Jun. 24, 2019, which claims the benefit of U.S.Provisional Applications No. 62/700,181 filed on Jul. 18, 2018, No.62/741,528 filed on Oct. 4, 2018, and Korean Patent Application No.10-2018-0076446 filed on Jul. 2, 2018, the contents of which are allhereby incorporated by reference herein in their entirety.

RELATED ART

Demand for high-resolution, high-quality images such as HD (HighDefinition) images and UHD (Ultra High Definition) images has beenincreasing in various fields. As the image data has high resolution andhigh quality, the amount of information or bits to be transmittedincreases relative to the legacy image data. Therefore, when image datais transmitted using a medium such as a conventional wired/wirelessbroadband line or image data is stored using an existing storage medium,the transmission cost and the storage cost thereof are increased.

Accordingly, there is a need for a highly efficient image compressiontechnique for effectively transmitting, storing, and reproducinginformation of high resolution and high quality images.

SUMMARY

This document provides a method and apparatus for enhancing image codingefficiency.

This document further provides a method and apparatus for enhancingimage coding efficiency based on intra prediction.

This document further provides a method and apparatus for enhancingefficiency of CCLM-based intra prediction.

This document further provides a method and apparatus for enhancing apipeline delay when implementing CCLM-based intra prediction inhardware.

This document further provides a method and apparatus for enhancingimage coding efficiency by downsampling neighboring luma referencesamples of a 1-sample line (hereinafter, ‘1-sample line’ means samplesof one line) when performing CCLM-based intra prediction.

In an aspect, a method for decoding a picture, performed by a decodingapparatus is provided. The method includes deriving neighboring chromareference samples for a chroma block when an intra prediction mode ofthe chroma block is a cross-component linear model (CCLM) mode; derivingneighboring luma reference samples of a luma block corresponding to thechroma block and luma samples in the luma block; deriving down-sampledneighboring luma reference samples and down-sampled luma samples bydown-sampling the neighboring luma reference samples and the lumasamples; deriving a linear model parameter based on the down-sampledneighboring luma reference samples and the neighboring chroma referencesamples; generating prediction samples for the chroma block based on thelinear model parameter and the down-sampled luma samples of the lumablock; and reconstructing the chroma block based on the predictionsamples for the chroma block, wherein the neighboring luma referencesamples include top neighboring luma reference samples located at theupper side of a top boundary of the luma block and left neighboring lumareference samples located at the left side of a left boundary of theluma block, and wherein when the top boundary of the luma block overlapswith a boundary of a coding tree unit (CTU), the number of the topneighboring luma reference samples used for deriving the down-sampledneighboring luma reference samples among the neighboring luma referencesamples is less than that of the left neighboring luma reference samplesused for deriving the down-sampled neighboring luma reference samples.

In another aspect, a decoding apparatus for performing picture decodingis provided. When an intra prediction mode of a chroma block is a CCLMmode, the decoding apparatus includes an adder for deriving neighboringchroma reference samples for the chroma block and neighboring lumareference samples of a luma block corresponding to the chroma block andluma samples in the luma block; and a prediction unit for derivingdown-sampled neighboring luma reference samples and down-sampled lumasamples by downsampling the neighboring luma reference samples and theluma samples, deriving a linear model parameter based on thedown-sampled neighboring luma reference samples and the neighboringchroma reference samples, and generating prediction samples for thechroma block based on the linear model parameter and the down-sampledluma samples of the luma block, wherein the adder reconstructs thechroma block based on the prediction samples for the chroma block, andthe neighboring luma reference samples include top neighboring lumareference samples located at the upper side of a top boundary of theluma block and left neighboring luma reference samples located at theleft side of the left boundary of the luma block, and when the topboundary of the luma block overlaps with a boundary of a coding treeunit (CTU), the number of the top neighboring luma reference samplesused for deriving the down-sampled neighboring luma reference samplesamong the neighboring luma reference samples is less than that of theleft neighboring luma reference samples used for deriving thedown-sampled neighboring luma reference samples.

In another aspect, a method for encoding a picture performed by anencoding apparatus is provided. The method includes deriving neighboringchroma reference samples for a chroma block when an intra predictionmode of the chroma block is a cross-component linear model (CCLM) mode;deriving neighboring luma reference samples of a luma blockcorresponding to the chroma block and luma samples in the luma block;deriving down-sampled neighboring luma reference samples anddown-sampled luma samples by down-sampling the neighboring lumareference samples and the luma samples; deriving a linear modelparameter based on the down-sampled neighboring luma reference samplesand the neighboring chroma reference samples; generating predictionsamples for the chroma block based on the linear model parameter and thedown-sampled luma samples of the luma block; deriving residual samplesfor the chroma block based on the prediction samples for the chromablock; and encoding picture information including information about theresidual samples, wherein the neighboring luma reference samples includetop neighboring luma reference samples located at the upper side of atop boundary of the luma block, and left neighboring luma referencesamples located at the left side of a left boundary of the luma block,and wherein when the top boundary of the luma block overlaps with aboundary of a coding tree unit (CTU), the number of the top neighboringluma reference samples used for deriving the down-sampled neighboringluma reference samples among the neighboring luma reference samples isless than that of the left neighboring luma reference samples used forderiving the down-sampled neighboring luma reference samples.

In another aspect, an encoding apparatus for performing picture encodingis provided. When an intra prediction mode of a chroma block is a CCLMmode, the encoding apparatus includes an adder for deriving neighboringchroma reference samples for the chroma block and neighboring lumareference samples of a luma block corresponding to the chroma block andluma samples in the luma block; a prediction unit for derivingdown-sampled neighboring luma reference samples and down-sampled lumasamples by downsampling the neighboring luma reference samples and theluma samples, deriving a linear model parameter based on thedown-sampled neighboring luma reference samples and the neighboringchroma reference samples, and generating prediction samples for thechroma block based on the linear model parameter and down-sampled lumasamples of the luma block; a residual processor for deriving residualsamples for the chroma block based on the prediction samples for thechroma block; and an entropy encoding unit for encoding pictureinformation including information about the residual samples, whereinthe neighboring luma reference samples include top neighboring lumareference samples located at the upper side of a top boundary of theluma block and left neighboring luma reference samples located at theleft side of the left boundary of the luma block, and when the topboundary of the luma block overlaps with a boundary of a coding treeunit (CTU), the number of the top neighboring luma reference samplesused for deriving the down-sampled neighboring luma reference samplesamong the neighboring luma reference samples is less than that of theleft neighboring luma reference samples used for deriving thedown-sampled neighboring luma reference samples.

In another aspect, a storage medium readable by a decoder for storingpicture information generated by a picture encoding method is provided,and the method of encoding the picture includes deriving neighboringchroma reference samples for a chroma block when an intra predictionmode of the chroma block is a cross-component linear model (CCLM) mode;deriving neighboring luma reference samples of a luma blockcorresponding to the chroma block and luma samples in the luma block;deriving down-sampled neighboring luma reference samples anddown-sampled luma samples by down-sampling the neighboring lumareference samples and the luma samples; deriving a linear modelparameter based on the down-sampled neighboring luma reference samplesand the neighboring chroma reference samples; generating predictionsamples for the chroma block based on the linear model parameter and thedown-sampled luma samples of the luma block; deriving residual samplesfor the chroma block based on the prediction samples for the chromablock; and encoding picture information including information about theresidual samples, wherein the neighboring luma reference samples includetop neighboring luma reference samples located at the upper side of atop boundary of the luma block, and left neighboring luma referencesamples located at the left side of a left boundary of the luma block,and wherein when the top boundary of the luma block overlaps with aboundary of a coding tree unit (CTU), the number of the top neighboringluma reference samples used for deriving the down-sampled neighboringluma reference samples among the neighboring luma reference samples isless than that of the left neighboring luma reference samples used forderiving the down-sampled neighboring luma reference samples.

According to this document, overall image/video compression efficiencycan be improved.

According to this document, efficiency of image coding based on intraprediction can be improved.

According to this document, efficiency of CCLM-based intra predictioncan be improved.

According to this document, pipeline delay can be improved whenimplementing CCLM-based intra prediction in hardware.

According to this document, when performing CCLM-based intra prediction,by down-sampling neighboring luma reference samples of a 1-sample line,image coding efficiency can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically illustrating a configuration of anencoding apparatus according to an embodiment.

FIG. 2 is a diagram schematically illustrating a configuration of adecoding apparatus according to an embodiment.

FIG. 3 is a diagram illustrating CCLM-based intra prediction accordingto an embodiment.

FIG. 4 is a diagram illustrating CCLM-based intra prediction accordingto another embodiment.

FIG. 5 is a flowchart illustrating an intra prediction process based onCCLM according to an embodiment.

FIG. 6 is a diagram illustrating an example of a hardware pipeline ofCCLM-based intra prediction according to an embodiment.

FIG. 7 is a diagram illustrating an example of a hardware pipeline ofCCLM-based intra prediction according to another embodiment.

FIG. 8 is a flowchart illustrating an intra prediction process based ona CCLM according to another embodiment.

FIG. 9 is a diagram illustrating an example of a hardware pipeline ofCCLM-based intra prediction according to another embodiment.

FIG. 10 is a flowchart illustrating an intra prediction process based ona CCLM according to another embodiment.

FIG. 11 is a diagram illustrating an intra prediction process based on aCCLM according to another embodiment.

FIG. 12 is a flowchart illustrating an intra prediction process based ona CCLM according to another embodiment.

FIG. 13 is a flowchart illustrating an operation of an encodingapparatus according to an embodiment.

FIG. 14 is a block diagram illustrating a configuration of an encodingapparatus according to an embodiment.

FIG. 15 is a flowchart illustrating an operation of a decoding apparatusaccording to an embodiment.

FIG. 16 is a block diagram illustrating a configuration of a decodingapparatus according to an embodiment.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

According to an embodiment of this document, a method for decoding apicture, performed by a decoding apparatus is provided. The methodincludes deriving neighboring chroma reference samples for a chromablock when an intra prediction mode of the chroma block is across-component linear model (CCLM) mode; deriving neighboring lumareference samples of a luma block corresponding to the chroma block andluma samples in the luma block; deriving down-sampled neighboring lumareference samples and down-sampled luma samples by down-sampling theneighboring luma reference samples and the luma samples; deriving alinear model parameter based on the down-sampled neighboring lumareference samples and the neighboring chroma reference samples;generating prediction samples for the chroma block based on the linearmodel parameter and the down-sampled luma samples of the luma block; andreconstructing the chroma block based on the prediction samples for thechroma block, wherein the neighboring luma reference samples include topneighboring luma reference samples located at the upper side of a topboundary of the luma block and left neighboring luma reference sampleslocated at the left side of a left boundary of the luma block, andwherein when the top boundary of the luma block overlaps with a boundaryof a coding tree unit (CTU), the number of the top neighboring lumareference samples used for deriving the down-sampled neighboring lumareference samples among the neighboring luma reference samples is lessthan that of the left neighboring luma reference samples used forderiving the down-sampled neighboring luma reference samples.

This document can be modified in various forms, and specific embodimentsthereof will be described and shown in the drawings. However, theembodiments are not intended for limiting this document. The terms usedin the following description are used to merely describe specificembodiments, but are not intended to limit this document. An expressionof a singular number includes an expression of the plural number, solong as it is clearly read differently. The terms such as “include” and“have” are intended to indicate that features, numbers, steps,operations, elements, components, or combinations thereof used in thefollowing description exist and it should be thus understood that thepossibility of existence or addition of one or more different features,numbers, steps, operations, elements, components, or combinationsthereof is not excluded.

Each of the components in the drawings described in this document areshown independently for the convenience of description regardingdifferent characteristic functions, and do not mean that the componentsare implemented in separate hardware or separate software. For example,two or more of each configuration may be combined to form oneconfiguration, or one configuration may be divided into a plurality ofconfigurations. Embodiments in which each configuration is integratedand/or separated are also included in the scope of this document withoutdeparting from the spirit of this document.

The following description relates to video/image coding. For example,the methods/embodiments disclosed in this document may be applied to amethod disclosed in the versatile video coding (VVC), the EVC (essentialvideo coding) standard, the AOMedia Video 1 (AV1) standard, the 2ndgeneration of audio video coding standard (AVS2), or the next generationvideo/image coding standard (ex. H.267 or H.268, etc.).

Hereinafter, exemplary embodiments of this document will be described indetail with reference to the accompanying drawings. Hereinafter, thesame reference numerals are used for the same components in thedrawings, and redundant description of the same components is omitted.

In this document, video may refer to a series of images over time.Picture generally refers to a unit representing one image in a specifictime zone, and a slice is a unit constituting part of a picture incoding. One picture may be configured with a plurality of slices, and ifnecessary, the picture and the slice may be mixed with each other.

A pixel or a pel may mean a smallest unit constituting one picture (orimage). Also, ‘sample’ may be used as a term corresponding to a pixel. Asample may generally represent a pixel or a value of a pixel, and mayrepresent only a pixel/pixel value of a luma component or only apixel/pixel value of a chroma component.

A unit may represent a basic unit of image processing. The unit mayinclude at least one of a specific region of the picture and informationrelated to the region. One unit may include one luma block and twochroma blocks. The unit may be used interchangeably with terms such asblock or area in some cases. In a general case, an M×N block may includesamples or a set of transform coefficients of M columns and N rows.

FIG. 1 is a diagram schematically illustrating a configuration of avideo encoding apparatus to which this document may be applied.Hereinafter, an encoding/decoding apparatus may include a videoencoding/decoding apparatus and/or an image encoding/decoding apparatus,and the video encoding/decoding apparatus may be used as a conceptincluding an image encoding/decoding apparatus or the imageencoding/decoding apparatus may be used as a concept including a videoencoding/decoding apparatus.

Referring to FIG. 1, an (video) encoding apparatus 100 may include apicture partitioning module 105, a prediction module 110, a residualprocessing module 120, an entropy encoding module 130, an adder 140, afiltering module 150, and a memory 160. The residual processing module120 may include a subtractor 121, a transform module 122, a quantizationmodule 123, a rearrangement module 124, a dequantization module 125, andan inverse transform module 126.

The picture partitioning module 105 may divide an input picture into atleast one processing unit.

As an example, the processing unit may be referred to as a coding unit(CU). In this case, the coding unit may be recursively divided accordingto a quad-tree binary-tree (QTBT) structure from a largest coding unit(LCU). For example, one coding unit may be divided into a plurality ofcoding units of a deeper depth based on a quad tree structure, a binarytree structure, and/or a ternary tree structure. In this case, forexample, the quad tree structure may be first applied, and the binarytree structure and the ternary tree structure may be applied later.Alternatively, the binary tree structure/ternary tree structure may befirst applied. A coding procedure according to this document may beperformed based on a final coding unit that is no longer divided. Inthis case, a largest coding unit may be used immediately as a finalcoding unit based on coding efficiency according to an imagecharacteristic or the coding unit may be recursively divided into codingunits of deeper depths, as needed and thus a coding unit of an optimizedsize may be used as a final coding unit. Here, the coding procedure mayinclude a procedure such as prediction, transform, and reconstruction tobe described later.

As another example, the processing unit may include a coding unit (CU),a prediction unit (PU), or a transform unit (TU). The coding unit may besplit from a largest coding unit (LCU) into coding units of deeperdepths according to a quad tree structure. In this case, the largestcoding unit may be immediately used as a final coding unit based oncoding efficiency according to an image characteristic, or the codingunit may be recursively divided into coding units of lower depths, asneeded and thus a coding unit of an optimized size may be used as afinal coding unit. When a smallest coding unit (SCU) is set, the codingunit cannot be split into smaller coding units than the SCU. Here, thefinal coding unit means a coding unit to be the basis of beingpartitioned or split into a prediction unit or a transform unit. Theprediction unit is a unit to be partitioned from the coding unit and maybe a unit of sample prediction. In this case, the prediction unit may bedivided into sub blocks. The transform unit may be divided according toa quad tree structure from the coding unit and may be a unit forderiving a transform coefficient and/or a unit for deriving a residualsignal from the transform coefficient. Hereinafter, a coding unit may bereferred to as a coding block (CB), a prediction unit may be referred toas a prediction block (PB), and a transform unit may be referred to as atransform block (TB). The prediction block or the prediction unit maymean a specific area of a block form within a picture and include anarray of prediction samples. Further, the transform block or thetransform unit may mean a specific area of a block form within a pictureand include an array of transform coefficients or residual samples.

The prediction module 110 may perform prediction on a block to beprocessed (hereinafter, may mean a current block or a residual block)and generate a predicted block including prediction samples for thecurrent block. A unit of prediction performed by the prediction module110 may be a coding block, a transform block, or a prediction block.

The prediction module 110 may determine whether intra prediction orinter prediction is applied to the current block. As an example, theprediction module 110 may determine whether intra prediction or interprediction is applied in a CU unit.

In the case of intra prediction, the prediction module 110 may derive aprediction sample for a current block based on a reference sampleoutside the current block in a picture (hereinafter, a current picture)to which the current block belongs. In this case, the prediction module110 may (i) derive a prediction sample based on average or interpolationof neighboring reference samples of the current block and (ii) derivethe prediction sample based on a reference sample present in a specific(prediction) direction for a prediction sample of neighboring referencesamples of the current block. A case of (i) may be referred to as anon-directional mode or a non-angle mode, and a case of (ii) may bereferred to as a directional mode or an angular mode. In intraprediction, the prediction mode may have, for example, 33 directionalprediction modes and at least two non-directional modes. Thenon-directional mode may include a DC prediction mode and a plannermode. The prediction module 110 may determine a prediction mode appliedto the current block using the prediction mode applied to theneighboring block.

In the case of inter prediction, the prediction module 110 may derive aprediction sample for a current block based on a sample specified by amotion vector on a reference picture. The prediction module 110 mayapply any one of a skip mode, a merge mode, and a motion vectorprediction (MVP) mode to derive a prediction sample for the currentblock. In the case of the skip mode and the merge mode, the predictionmodule 110 may use motion information of the neighboring block as motioninformation of the current block. In the case of the skip mode, unlikethe merge mode, the difference (residual) between the prediction sampleand the original sample is not transmitted. In the case of the MVP mode,by using a motion vector predictor of the current block using a motionvector of the neighboring block as a motion vector predictor, a motionvector of the current block may be derived.

In the case of inter prediction, the neighboring block may include aspatial neighboring block existing in the current picture and a temporalneighboring block present in the reference picture. A reference pictureincluding the temporal neighboring block may be referred to as acollocated picture (colPic). The motion information may include a motionvector and a reference picture index. Information such as predictionmode information and motion information may be (entropy) encoded andoutput in the form of a bitstream.

When motion information of the temporal neighboring block is used in theskip mode and the merge mode, the highest picture on a reference picturelist may be used as the reference picture. Reference pictures includedin the reference picture list may be arranged based on a difference in apicture order count (POC) between a current picture and a correspondingreference picture. The POC corresponds to display order of pictures andmay be distinguished from the coding order.

The subtractor 121 generates a residual sample, which is a differencebetween the original sample and the prediction sample. When the skipmode is applied, residual samples may not be generated, as describedabove.

The transform module 122 transforms the residual sample in transformblock units to generate transform coefficients. The transform module 122may perform transform according to a size of the transform block and aprediction mode applied to the coding block or the prediction block thatspatially overlaps with the transform block. For example, when intraprediction is applied to the coding block or the prediction block thatoverlaps with the transform block, and the transform block is a 4×4residual array, the residual sample is transformed using a discrete sinetransform (DST) kernel, and in other cases, the residual sample may betransformed using a discrete cosine transform (DCT) kernel.

The quantization module 123 may quantize transform coefficients togenerate quantized transform coefficients.

The rearrangement module 124 rearranges the quantized transformcoefficients. The rearrangement module 124 may rearrange quantizedtransform coefficients of a block form into a one-dimensional vectorform through a coefficient scanning method. Here, the rearrangementmodule 124 has been described in a separate configuration, but therearrangement module 124 may be part of the quantization module 123.

The entropy encoding module 130 may perform entropy encoding for thequantized transform coefficients. Entropy encoding may include, forexample, encoding methods such as exponential Golomb, context-adaptivevariable length coding (CAVLC), context-adaptive binary arithmeticcoding (CABAC), and the like. The entropy encoding module 130 may encodeaccording to entropy encoding or a predetermined method together with orseparately from information (e.g., a value of a syntax element)necessary for video reconstruction other than the quantized transformcoefficients. The encoded information may be transmitted or stored inunits of network abstraction layer (NAL) units in the form of abitstream. The bitstream may be transmitted through a network or may bestored in a digital storage medium. Here, the network may include abroadcasting network and/or a communication network, and the digitalstorage medium may include various storage media such as a universalserial bus (USB), secure digital (SD), compact disc (CD), digital videodisc (DVD), Blu-ray, hard disk drive (HDD), solid state drive (SSD) andthe like.

The dequantization module 125 inverse quantizes quantized values(quantized transform coefficients) in the quantization module 123, andthe inverse transform module 126 inverse transforms inverse quantizedvalues in the dequantization module 125 to generate a residual sample.

The adder 140 reconstructs the picture by combining the residual sampleand the predictive sample. The residual sample and the prediction samplemay be added in units of blocks to generate a reconstructed block. Here,the adder 140 has been described in a separate configuration, but theadder 140 may be part of the prediction module 110. The adder 140 mayalso be referred to as a reconstruction module or a reconstruction blockgenerator.

The filtering module 150 may apply a deblocking filter and/or a sampleadaptive offset to the reconstructed picture. Through deblockingfiltering and/or sample adaptive offset, artifacts at a block boundarywithin the reconstructed picture or distortion in a quantization processmay be corrected. The sample adaptive offset may be applied in a sampleunit and may be applied after a process of deblocking filtering iscompleted. The filtering module 150 may apply an adaptive loop filter(ALF) to the reconstructed picture. The ALF may be applied to thereconstructed picture after the deblocking filter and/or sample adaptiveoffset is applied.

The memory 160 may store reconstructed pictures (decoded pictures) orinformation necessary for encoding/decoding. Here, the reconstructedpicture may be a reconstructed picture in which the filtering process iscompleted by the filtering module 150. The stored reconstructed picturemay be used as a reference picture for (inter) prediction of anotherpicture. For example, the memory 160 may store (reference) pictures usedfor inter prediction. In this case, pictures used for inter predictionmay be designated by a reference picture set or a reference picturelist.

FIG. 2 is a diagram schematically illustrating a configuration of avideo/image decoding apparatus to which this document may be applied.Hereinafter, the video decoding apparatus may include an image decodingapparatus.

Referring to FIG. 2, the video decoding apparatus 200 may include anentropy decoding module 210, a residual processing module 220, aprediction module 230, an adder 240, a filtering module 250, and amemory 260. Here, the residual processing module 220 may include arearrangement module 221, a dequantization module 222, and an inversetransform module 223. Further, although not shown, the video decodingapparatus 200 may include a receiver that receives bitstream includingvideo information. The receiver may be configured as a separate moduleor may be included in the entropy decoding module 210.

When bitstream including video/image information is input, the videodecoding apparatus 200 may reconstruct a video/image/picture tocorrespond to a process in which video/image information is processed inthe video encoding apparatus.

For example, the video decoding apparatus 200 may perform video decodingusing a processing unit applied in the video encoding apparatus. Thus, aprocessing unit block of video decoding may be, for example, a codingunit, and in another example, the processing unit block of videodecoding may be a coding unit, a prediction unit, or a transform unit.The coding unit may be split according to a quad tree structure, abinary tree structure, and/or a ternary tree structure from the largestcoding unit.

The prediction unit and the transform unit may be further used in somecases, and in this case, the prediction block is a block derived orpartitioned from the coding unit and may be a unit of sample prediction.In this case, the prediction unit may be split into subblocks. Thetransform unit may be split according to the quad tree structure fromthe coding unit, and may be a unit for deriving a transform coefficientor a unit for deriving a residual signal from a transform coefficient.

The entropy decoding module 210 may parse bitstream and outputinformation necessary for video reconstruction or picturereconstruction. For example, the entropy decoding module 210 may decodeinformation in the bitstream based on a coding method such asexponential Golomb coding, CAVLC, or CABAC and output values of syntaxelements required for video reconstruction and quantized values oftransform coefficients for residuals.

More specifically, the CABAC entropy decoding method may includereceiving a bin corresponding to each syntax element in a bitstream,determining a context model using to-be-decoded syntax elementinformation, decoding information of a neighboring block and ato-be-decoded block, or information of a symbol/bin decoded in aprevious step, and predicting a probability of occurrence of a binaccording to the determined context model and thus performing arithmeticdecoding of the bin to generate a symbol corresponding to a value ofeach syntax element. In this connection, after determining the contextmodel, the CABAC entropy decoding method may further include a step ofupdating the context model using the information of the decodedsymbol/bin to determine a context model of the next symbol/bin.

Information related to prediction among information decoded by theentropy decoding module 210 may be provided to the prediction module230, and a residual value in which entropy decoding is performed by theentropy decoding module 210, i.e., the quantized transform coefficientmay be input to the rearrangement module 221.

The rearrangement module 221 may rearrange the quantized transformcoefficients in a two-dimensional block form. The rearrangement module221 may perform rearrangement to correspond to coefficient scanningperformed by the encoding apparatus. Here, the rearrangement module 221has been described in a separate configuration, but the rearrangementmodule 221 may be part of the dequantization module 222.

The dequantization module 222 may dequantize the quantized transformcoefficients based on the (inverse) quantization parameter to output thetransform coefficients. In this case, information for deriving aquantization parameter may be signaled from the encoding apparatus.

The inverse transform unit 223 may inversely transform transformcoefficients to derive residual samples.

The prediction module 230 may perform prediction of a current block andgenerate a predicted block including prediction samples for the currentblock. A unit of prediction performed by the prediction module 230 maybe a coding block, a transform block, or a prediction block.

The prediction module 230 may determine whether to apply intraprediction or inter prediction based on information about theprediction. In this case, a unit for determining one of intra predictionand inter prediction to apply and a unit for generating a predictionsample may be different. Further, a unit for generating a predictionsample in inter prediction and intra prediction may also be different.For example, whether to apply inter prediction or intra prediction maybe determined in units of a CU. Further, for example, in interprediction, a prediction mode may be determined and a prediction samplemay be generated in PU units, and in intra prediction, a prediction modemay be determined in PU units and a prediction sample may be generatedin TU units.

In the case of intra prediction, the prediction module 230 may derive aprediction sample for a current block based on the neighbor referencesamples in a current picture. By applying a directional mode or anon-directional mode based on the neighbor reference samples of thecurrent block, the prediction module 230 may derive the predictionsample for the current block. In this case, the prediction mode to beapplied to the current block may be determined using the intraprediction mode of the neighboring block.

In the case of inter prediction, the prediction module 230 may derivethe prediction sample for the current block based on the samplespecified on the reference picture by a motion vector on the referencepicture. The prediction module 230 may apply one of a skip mode, a mergemode, and an MVP mode to derive a prediction sample for the currentblock. In this case, motion information required for inter prediction ofthe current block provided by the video encoding apparatus, for example,information about a motion vector, a reference picture index, and thelike may be obtained or derived based on information about theprediction.

In case of the skip mode and the merge mode, motion information of theneighboring block may be used as motion information of the currentblock. In this case, the neighboring block may include a spatialneighboring block and a temporal neighboring block.

The prediction module 230 may construct a merge candidate list withmotion information of available neighboring blocks and use informationindicated by a merge index on the merge candidate list as a motionvector of the current block. The merge index may be signaled from theencoding apparatus. The motion information may include a motion vectorand a reference picture. When motion information of the temporalneighboring block is used in the skip mode and the merge mode, a highestpicture on the reference picture list may be used as a referencepicture.

In case of the skip mode, unlike the merge mode, the difference(residual) between the prediction sample and the original sample is nottransmitted.

In case of the MVP mode, the motion vector of the current block may bederived using the motion vector of the neighboring block as a motionvector predictor. In this case, the neighboring block may include aspatial neighboring block and a temporal neighboring block.

For example, when the merge mode is applied, a merge candidate list maybe generated using a motion vector of a reconstructed spatialneighboring block and/or a motion vector corresponding to a Col block,which is a temporal neighboring block. In the merge mode, the motionvector of the candidate block selected from the merge candidate list isused as the motion vector of the current block. Information about theprediction may include a merge index indicating a candidate block havingan optimal motion vector selected from candidate blocks included in themerge candidate list. In this case, the prediction module 230 may derivea motion vector of the current block using the merge index.

As another example, when the Motion Vector Prediction (MVP) mode isapplied, a motion vector predictor candidate list may be generated usinga motion vector corresponding to a motion vector of a reconstructedspatial neighboring block and/or a motion vector corresponding to a Colblock, which is a temporal neighboring block. That is, a motion vectorof the reconstructed spatial neighboring block and/or a motion vectorcorresponding to the Col vector, which is a temporal neighboring block,may be used as a motion vector candidate. The information about theprediction may include a prediction motion vector index indicating anoptimal motion vector selected from the motion vector candidatesincluded in the list. In this case, the prediction module 230 may selecta predicted motion vector of the current block from the motion vectorcandidates included in the motion vector candidate list using the motionvector index. The prediction unit of the encoding apparatus may obtain amotion vector difference (MVD) between the motion vector of the currentblock and the motion vector predictor and encode the MVD to output theMVD in a bitstream form. That is, the MVD may be obtained by subtractingthe motion vector predictor from the motion vector of the current block.In this case, the prediction module 230 may obtain a motion vectordifference included in information about the prediction, and derive themotion vector of the current block by adding the motion vectordifference and the motion vector predictor. The prediction unit may alsoobtain or derive a reference picture index or the like indicating areference picture from the information about the prediction.

The adder 240 may add the residual sample and the predictive sample toreconstruct the current block or the current picture. The adder 240 mayadd the residual sample and the predictive sample in block units toreconstruct the current picture. When the skip mode is applied, theresidual is not transmitted and thus the prediction sample may be areconstruction sample. Here, the adder 240 has been described in aseparate configuration, but the adder 240 may be part of the predictionmodule 230. The adder 240 may also be referred to as a reconstructionmodule or a reconstruction block generator.

The filtering module 250 may apply deblocking filtering sample adaptiveoffset and/or an ALF to the reconstructed picture. In this case, thesample adaptive offset may be applied in units of samples and be appliedafter deblocking filtering. The ALF may be applied after deblockingfiltering and/or sample adaptive offset.

The memory 260 may store reconstructed pictures (decoded pictures) orinformation necessary for decoding. Here, the reconstructed picture maybe a reconstructed picture in which a filtering procedure is completedby the filtering module 250. For example, the memory 260 may storepictures used for inter prediction. In this case, pictures used forinter prediction may be designated by a reference picture set or areference picture list. The reconstructed picture may be used as areference picture for another picture. Further, the memory 260 mayoutput the reconstructed picture in output order.

As described above, in performing video coding, prediction is performedto increase compression efficiency. Thereby, a predicted block includingprediction samples of the current block, which is a coding target block,may be generated. Here, the predicted block includes prediction samplesin a spatial domain (or pixel domain). The predicted block is equallyderived in the encoding apparatus and the decoding apparatus, and theencoding apparatus may signal information (residual information) aboutresidual between the original block and the predicted block instead ofan original sample value itself of the original block to the decodingapparatus to increase image coding efficiency. The decoding apparatusmay derive a residual block including residual samples based on theresidual information, generate a reconstructed block includingreconstructed samples by adding the residual block and the predictedblock, and generate a reconstructed picture including reconstructedblocks.

The residual information may be generated through transform andquantization procedures. For example, the encoding apparatus may derivea residual block between the original block and the predicted block,perform a transform procedure of residual samples (residual samplearray) included in the residual block to derive transform coefficients,perform a quantization procedure of the transform coefficients to derivethe quantized transform coefficients, and signal related residualinformation to the decoding apparatus (through bitstream). Here, theresidual information may include information such as value informationof the quantized transform coefficients, position information, atransform scheme, a transform kernel, and a quantization parameter. Thedecoding apparatus may perform an inverse quantization/inversetransformation procedure and derive residual samples (or residualblocks) based on the residual information. The decoding apparatus maygenerate a reconstructed picture based on the predicted block and theresidual block. The encoding apparatus may inverse quantize/inversetransform quantized transform coefficients for reference for interprediction of a subsequent picture to derive the residual block, andgenerate a reconstructed picture based on the residual block.

FIG. 3 is a diagram illustrating CCLM-based intra prediction accordingto an embodiment.

According to an embodiment, when performing intra encoding for a chromaimage, a cross-component linear model (CCLM) mode may be used as one ofprediction modes. The CCLM is a method of predicting a sample value (orpixel value) of a chroma image (or chroma block) based on a sample valueof a reconstructed luma image (or luma block) and is a method using acharacteristic in which correlation between a luma image and a chromaimage is high.

In one embodiment, intra prediction based on the CCLM mode of Cb and Crchroma images may be performed based on Equation 1.Pred_(c)(x,y)=Rec′_(L)(x,y)+β  [Equation 1]

Pred_(c)(x,y) means a sample value of the Cb or Cr chroma image to bepredicted, Rec′_(L)(x,y) means a sample value of the reconstructed lumablock adjusted to a chroma block size, and (x,y) means a coordinate ofthe sample. In a color format of 4:2:0, because a size of the luma imageis two times larger than that of the chroma image, Rec′_(L) of thechroma block size should be generated through downsampling, and thus asample of a luma image to be used in the sample value Pred_(c)(x,y) ofthe chroma image may consider a neighboring sample in addition toRec_(L)(2x, 2y). In one example, Rec′_(L)(x,y) may be derived based on asample value of six neighboring samples, as shown in Equation 2.Rec′_(L)(x,y)=2×Rec_(L)(2x,2y)+2×Rec_(L)(2x,2y+1)+Rec_(L)(2x−1,2y)+Rec_(L)(2x+1,2y)+Rec_(L)(2x−1,2y+1)+Rec_(L)(2x+1,2y+1)+4)>>3  [Equation2]

In one embodiment, the linear model parameters α and β used whenapplying the CCLM mode may be derived, as shown in Equation 3 based onthe cross-correlation between the Cb or Cr neighboring chroma referencesample area or template and the neighboring luma reference sample areaand the difference between the average values, as in a light shaded areaof FIG. 3.

$\begin{matrix}{{\alpha = \frac{{M\left( {{t_{L}\left( {x,y} \right)} - {M\left( t_{L} \right)}} \right)} \times {M\left( {{t_{C}\left( {x,y} \right)} - {M\left( t_{C} \right)}} \right)}}{{M\left( {{t_{L}\left( {x,y} \right)} - {M\left( t_{L} \right)}} \right)} \times {M\left( {{t_{L}\left( {x,y} \right)} - {M\left( t_{L} \right)}} \right)}}},} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack \\{\beta = {{M\left( t_{C} \right)} - {\alpha\;{M\left( t_{L} \right)}}}} & \;\end{matrix}$

In Equation 3, t_(L) means a neighboring luma reference sample of theluma block corresponding to a current chroma block, t_(CL) means aneighboring reference chroma sample of the chroma block to whichencoding is currently applied, and (x,y) means a sample position.Further, M(A) means the average of samples A in the reference samplearea. A reference sample of the luma block may also be derived based ondownsampling, as in Equation 3.

In another embodiment, when an intra prediction mode of the Cr chromaimage is not the CCLM mode, intra prediction may be performed based on abasic intra prediction mode (e.g., the basic intra prediction mode mayinclude a DC mode, a PLANAR mode, and other directional intra predictionmodes). Thereafter, a CCLM may be applied between the Cb chroma imageand the Cr chroma image, and the result in which the CCLM is applied maybe reflected to the existing prediction block, as shown in Equation 3.In this case, it is possible to predict the other one using any one ofthe Cb and Cr chroma images, and in an example, when predicting the Crchroma image using the Cb chroma image, Equation 4 may be used.

$\begin{matrix}\left\{ \begin{matrix}{{{pred}_{Cr}\left( {x,y} \right)} = {{{pre\_ pred}_{Cr}\left( {x,y} \right)} +}} & {{{if}\mspace{14mu}\alpha} < 0} \\{\alpha - {{residual}_{Cr}\left( {x,y} \right)}} & \; \\{{{pred}_{Cr}\left( {x,y} \right)} = {{pre\_ pred}_{Cr}\left( {x,y} \right)}} & {otherwise}\end{matrix} \right. & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

In Equation 4, pred_(Cr) may mean a sample value of the final predictedCr chroma block, pre_pred_(Cr) may mean a sample value of the Cr chromablock predicted through a Cr chroma intra prediction mode other thanCCLM, and residual_(Cb) may mean a residual sample value (or residualpixel) of a Cb chroma image in which encoding has already beencompleted. Further, a may be calculated through cross-correlationbetween neighboring reference sample areas of the Cb and Cr chromablocks, as in Equation 1.

FIG. 4 is a diagram illustrating CCLM-based intra prediction accordingto another embodiment.

In an embodiment, when the upper reference sample line of the current(encoding) block is located at the boundary of another coding tree unit(CTU), reference sample downsampling may be performed using only onereference sample line directly above the current block, as in Equation 5instead of downsampling according to Equation 2.Rec′_(L)(x,y)=2×Rec_(L)(2x,2y+1)+Rec_(L)(2x−1,2y+1)+Rec_(L)(2x+1,2y+1)+2)>>2  [Equation5]

That is, as shown in Equation 2, downsampling of samples of the lumablock for a CCLM uses 6 samples (or pixels), but in one embodimentaccording to Equation 5, when the upper reference sample line of thecurrent block is located at the boundary of another CTU (or in case ofoverlapping with the boundary of another CTU), downsampling may beperformed using three samples of one reference sample line.

In hardware implementation, because a line buffer should include bothsample (or pixel) data and compression information of an entirehorizontal size of an image, the more the line buffer is used, thehigher the cost may be. According to the method proposed in anembodiment according to Equation 5, CCLM prediction may be performedusing only one line of line buffers at the upper CTU boundary and thusby reducing the line buffer used at the upper CTU boundary to (half),the cost of hardware implementation may be reduced.

Table 1 shows an example of experimental results when using the methodproposed in an embodiment according to FIG. 4 and Equation 5. In oneexample, an anchor of the experiment may be software in which CCLMtechnology is added to VTM1.0 and represents the result in which animage is encoded with all intra setting.

TABLE 1 Y U V Class A1 −0.01% 0.07% 0.00% Class A2 0.00% 0.06% 0.05%Class B 0.00% −0.02% 0.06% Class C 0.01% −0.11% 0.02% Class E 0.00%0.05% −0.10% Overall 0.00% 0.00% 0.01% Class D −0.03% -0.03% 0.04%

Referring to Table 1, when a CCLM mode is applied using the methodproposed in an embodiment according to Equation 5 of FIG. 4, comparedwith a case in which the CCLM mode is applied without using the methodproposed in an embodiment according to Equation 5, BD-rates of luma (Y)image 0.00%, chroma image Cb 0.00%, and chroma image Cr 0.01% may beobtained. That is, even when the CCLM mode is applied using the methodproposed in the embodiment according to Equation 5 of FIG. 4, it may bedetermined that an encoding loss does not occur.

In addition to the downsampling method proposed in Equation 5, in oneembodiment, the CCLM mode may be applied based on a downsampling methodsuch as Equation 6 or Equation 7.Rec′_(L)(x,y)=3×Rec_(L)(2x,2y+1)+Rec_(L)(2x+1,2y+1)+2)>>2  [Equation 6]Rec′_(L)(x,y)=Rec_(L)(2x,2y+1)  [Equation 7]

The downsampling methods according to Equations 2 and 5 to 7 may beapplied to the CCLM mode, which is a chroma intra prediction mode, andthe chroma block predicted through the CCLM mode may be used whenobtaining a residual image (or residual image) through the differencewith an original video in the encoding apparatus (or encoder) or may beused when obtaining a reconstructed image based on the sum with theresidual signal in the decoding apparatus (or decoder).

FIG. 5 is a flowchart illustrating an intra prediction process based ona CCLM according to an embodiment.

The decoding apparatus 200 according to an embodiment may generate adown-sampled luma template. In this case, the decoding apparatus 200 maydetermine whether a top boundary of a current block overlaps with aboundary of the CTU, generate an upper luma template based on 1-linedownsampling, if a top boundary of a current block overlaps with aboundary of the CTU, and generate an upper luma template based on 2-linedownsampling, if a top boundary of the current block does not overlapwith a boundary of the CTU. The decoding apparatus 200 may generate aleft luma template based on 2-line downsampling.

The decoding apparatus 200 according to an embodiment may derive α andβ, which are linear parameter model coefficients, generate down-sampledluma blocks, and perform CCLM prediction based on Equation 1.

FIG. 6 is a diagram illustrating an example of a hardware pipeline ofCCLM-based intra prediction according to an embodiment.

In one embodiment, for hardware pipelining optimization of intraprediction implemented with a CCLM, when downsampling a reconstructedluma image, a method of removing correlation between sample lines of ablock is proposed.

As described above, because a CCLM is a method of predicting a samplevalue of a chroma block using a sample value of a reconstructed lumablock, CCLM prediction of a chroma block may not be possible untilencoding of the luma block of the same position is completed.

Further, because downsampling of a reconstructed image is appliedthrough the method of Equation 2, correlation between sample lines ofthe block occurs and thus when implementing hardware of the CCLM, aclock delay may occur, as shown in FIG. 6.

FIG. 6 illustrates an example of a hardware pipeline according towhether CCLM prediction exists in 8×4 intra prediction blocks. As shownin FIG. 6, prediction and reconstruction of luma samples are performedin units of 4×1, and two clocks are required to decode 4×1 blocks, andthus total 16 clocks may be required to decode all luma samples of 8×4blocks. When there is no correlation between the luma and chroma images,the luma and chroma images are decoded at the same time and thusdecoding of the chroma images may be completed when a clock (8 clocks inthe example of FIG. 6) corresponding to a half of the luma image haselapsed. However, when CCLM algorithm is applied, after encoding of2-sample lines of the luma block is completed by correlation between thesample lines (fourth 4×1 lines in FIG. 6), a CCLM may be applied to a1-sample line of the chroma block and thus even when encoding of theluma block is terminated, four clock delays may occur for encoding thechroma block.

FIG. 7 is a diagram illustrating an example of a hardware pipeline ofCCLM-based intra prediction according to another embodiment.

In one embodiment, when downsampling a reconstructed luma image for aCCLM, a method of reducing a clock delay of a hardware pipeline byremoving correlation between sample lines may be provided. For thisreason, downsampling of the reconstructed luma block may be performedbased on Equation 8, Equation 9, Equation 10, or Equation 11.Rec′_(L)(x,y)=(2×Rec_(L)(2×,2y)+2×Rec_(L)(2×,2y+1)+Rec_(L)(2×−1,2y)+2)>>2  [Equation8]Rec′_(L)(x,y)=(2×Rec_(L)(2×,2y)+2×Rec_(L)(2×+1,2y)+Rec_(L)(2×−1,2y)+2)>>2  [Equation9]Rec′_(L)(x,y)=(3×Rec_(L)(2x,2y)+2×Rec_(L)(2x,2y+1))>>2  [Equation 10]Rec′_(L)(x,y)=(3×Rec_(L)(2x,2y)+2×Rec_(L)(2x+1,2y))>>2  [Equation 11]

Downsampling proposed in Equations 8 to 11 may be applied to areas otherthan the current neighboring reference sample area (or neighboringtemplate area).

In an embodiment, when downsampling is performed by removing correlationbetween sample lines, optimization of a hardware pipeline may beperformed as shown in FIG. 7. In the pipeline shown in FIG. 7, afterdecoding of one sample line of the luma block is completed, decoding ofthe chroma block may be performed immediately (second 4×1 line of FIG.7), and after decoding of the third sample line of the luma block isfinally completed (sixth 4×1 line of FIG. 7), decoding of the secondsample line of the chroma block may be performed. For decoding of thefourth sample line of the luma block, 4 clocks are required, and fordecoding of the second sample line of the chroma block, 4 clocks arealso required (2 clocks are required for each of chroma Cb and Cr) andthus decoding of luma and chroma blocks may finally be completed at thesame time without delay.

That is, a clock delay problem between the luma block and the chromablock can be solved through the method proposed in the presentembodiment, which can compensate for the disadvantages of hardwareimplementation of the CCLM.

The method proposed in the present patent may be used in a CCLM mode,which is a chroma intra prediction mode, and a chroma block predictedthrough the CCLM mode may be used when obtaining a residual imagethrough a difference from an original image in an encoder or may be usedwhen obtaining a reconstructed image through the sum with a residualsignal in a decoder.

FIG. 8 is a flowchart illustrating an intra prediction process based ona CCLM according to another embodiment.

The decoding apparatus 200 according to an embodiment may generate adown-sampled luma template and derive linear parameter modelcoefficients α and β.

The decoding apparatus 200 according to an embodiment may generate adown-sampled luma block. In this case, the decoding apparatus 200 maygenerate a luma sample based on 1-line downsampling.

The decoding apparatus 200 according to an embodiment may perform CCLMprediction based on Equation 1.

FIG. 9 is a diagram illustrating an example of a hardware pipeline ofCCLM-based intra prediction according to another embodiment.

In one embodiment, in order to optimize hardware pipelining of intraprediction in which a CCLM is implemented, when a horizontal length ofthe chroma block is 2 (or in the case of an image format of 4:4:4, whena horizontal length of the chroma block is 4), a method of not applyinga CCLM is proposed.

In intra prediction based on the embodiment of FIG. 7, when a CCLM modeis applied, hardware pipeline optimization may be performed, but when ahorizontal length of the luma block is 4, a clock delay may occur. FIG.9 illustrates an example of a hardware pipeline according to whetherthere is CCLM prediction in luma 4×4 intra prediction blocks. In FIG. 9,when downsampling the reconstructed luma block by applying the methodaccording to the embodiment of FIG. 7, correlation between sample linesis removed, but it may be determined that a delay of 2 clocks stilloccurs.

Therefore, in one embodiment, in order to increase hardwareimplementation affinity of the CCLM mode, when a horizontal length ofthe chroma block is 2 (or in the case of an image format of 4:4:4, whena horizontal length of the chroma block is 4), a method of not applyinga CCLM is proposed. That is, by removing a clock delay, which is aproblem in hardware implementation, a hardware incompatibility problemof a CCLM can be solved.

Table 2 shows actual experimental results when using both the methodproposed in the embodiment according to FIG. 7 and the embodimentaccording to FIG. 9. In one example, an anchor of the experiment may besoftware in which CCLM technology is added to VTM1.0, and represents theresult in which an image is encoded with all intra settings.

TABLE 2 Y U V Class A1 0.76% 4.49% 2.69% Class A2 0.13% 1.66% 1.08%Class B 0.08% 1.50% 2.23% Class C 0.39% 3.39% 4.02% Class E 0.02% 0.21%0.26% Overall 0.26% 2.23% 2.19% Class D 0.18% 2.97% 2.45%

As shown in Table 2, when using both the methods proposed in theembodiment according to FIG. 7 and in the embodiment according to FIG.9, a BD-rate of a luma image 0.26% and a chroma image Cb 2.23%/Cr 2.19%may be obtained. Because a CCLM is not applied in 4×N blocks, someperformance loss has occurred, but an effect due to a CCLM may be stillobtained. (Y −1.28%, Cb −8.03%, Cr −8.67% compared with VTM1.0)

The method proposed in the embodiment according to FIG. 7 and theembodiment according to FIG. 9 may be applied to a CCLM mode, which is achroma intra prediction mode, and a chroma block predicted through theCCLM mode may be used when obtaining a residual image through thedifference with an original image in the encoding apparatus (or encoder)or may be used when obtaining an image reconstructed based on the sumwith the residual signal in the decoding apparatus (or decoder).

FIG. 10 is a flowchart illustrating an intra prediction process based ona CCLM according to another embodiment.

When a horizontal length (or width) of the luma block is not 4, thedecoding apparatus 200 according to an embodiment may generate adown-sampled luma template, derive linear parameter model coefficients αand β, generate a down-sampled luma block, and perform CCLM predictionbased on Equation 1. Conversely, when a horizontal length (or width) ofthe luma block is 4, the above procedure (a procedure of generating adown-sampled luma template, deriving the linear parameter modelcoefficients α and β, generating down-sampled luma blocks, andperforming CCLM prediction based on Equation 1) may be omitted.

FIG. 11 is a diagram illustrating an intra prediction process based on aCCLM according to another embodiment.

In one embodiment, there is proposed a method of limiting a referencesample line when an upper reference sample line of a current (encoding)block is located at the boundary of another luma/chroma block separationunit block.

In an embodiment, in the case of an I slice, a luma block and a chromablock may be separated and encoding and decoding may be thus performed.In one example, the size of the luma/chroma block separation unit blockmay be 64×64, but the example is not limited thereto. The luma/chromablock separation unit block may be referred to as virtual pipeline dataunits (VPDUs).

For pipeline and parallel processing of hardware, correlation betweenluma/chroma block separation unit blocks needs to be minimized. In oneembodiment, for optimized parallel processing, at the top boundary ofthe luma/chroma block separation unit block, a reconstructed lumareference sample for a CCLM may not be used. That is, as shown in FIG.11, at the top boundary of the luma/chroma block separation unit block,by using only the left reference sample line for a CCLM, a redundancyissue in the hardware pipeline implementation can be prevented.

FIG. 12 is a flowchart illustrating an intra prediction process based ona CCLM according to another embodiment.

The decoding apparatus 200 according to an embodiment may generate adown-sampled luma template. In this case, the decoding apparatus 200 maydetermine whether the top boundary of the current block overlaps withthe boundary of the luma/chroma block separation unit block, and if thetop boundary of the current block overlaps with the boundary of theluma/chroma block separation unit block, the decoding apparatus 200 maynot generate an upper luma template, and if the top boundary of thecurrent block does not overlap with the boundary of the luma/chromablock separation unit block, the decoding apparatus 200 may generate anupper luma template based on 2-line downsampling. The decoding apparatus200 may generate a left luma template based on 2-line downsampling.

The decoding apparatus 200 according to an embodiment may derive α andβ, which are linear parameter model coefficients, generate down-sampledluma blocks, and perform CCLM prediction based on Equation 1.

FIG. 13 is a flowchart illustrating an operation of an encodingapparatus according to an embodiment, and FIG. 14 is a block diagramillustrating a configuration of an encoding apparatus according to anembodiment.

The encoding apparatus according to FIGS. 13 and 14 may performoperations corresponding to the decoding apparatus according to FIGS. 15and 16 to be described later. Accordingly, contents to be describedlater with reference to FIGS. 15 and 16 may be similarly applied to theencoding apparatus according to FIGS. 13 and 14.

Each step disclosed in FIG. 13 may be performed by the encodingapparatus 100 disclosed in FIG. 1. More specifically, S1300 and S1310may be performed by the adder 140 disclosed in FIG. 1, S1320 to S1340may be performed by the prediction module 110 disclosed in FIG. 1, S1350may be performed by the residual processing module 120 disclosed in FIG.1, and S1360 may be performed by the entropy encoding module 130disclosed in FIG. 1. In addition, operations according to S1300 to S1360are performed based on some of the contents described above withreference to FIGS. 3 to 12. Accordingly, a detailed description repeatedwith those described above with reference to FIGS. 1 and 3 to 12 will beomitted or simply described.

As shown in FIG. 14, an encoding apparatus according to an embodimentmay include a prediction module 110, a residual processing module 120,an entropy encoding module 130, and an adder 140. However, in somecases, all of the components shown in FIG. 14 may not be essentialcomponents of the encoding apparatus, and the encoding apparatus may beimplemented by more or less components than those illustrated in FIG.14. For example, the encoding apparatus may further include a memory160.

In the encoding apparatus according to an embodiment, the predictionmodule 110, the residual processing module 120, the entropy encodingmodule 130, and the adder 140 may be each implemented into separatechips or at least two components may be implemented through one chip.

When an intra prediction mode of the chroma block is a CCLM mode, theencoding apparatus according to an embodiment may derive neighboringchroma reference samples for the chroma block (S1300). Morespecifically, when an intra prediction mode of the chroma block is aCCLM mode, the adder 140 of the encoding apparatus may deriveneighboring chroma reference samples for the chroma block.

The encoding apparatus according to an embodiment may derive neighboringluma reference samples of the luma block corresponding to the chromablock and luma samples in the luma block (S1310). More specifically, theprediction module 110 of the encoding apparatus may derive neighboringluma reference samples of the luma block corresponding to the chromablock and luma samples in the luma block.

The encoding apparatus according to an embodiment may down-sample theneighboring luma reference samples and the luma samples to derive thedown-sampled neighboring luma reference samples and the down-sampledluma samples (S1320). More specifically, the prediction module 110 ofthe encoding apparatus may down-sample the neighboring luma referencesamples and the luma samples to derive the down-sampled neighboring lumareference samples and the down-sampled luma samples.

In one embodiment, when the top boundary of the luma block overlaps witha boundary of a coding tree unit (CTU), the number of the topneighboring luma reference samples used for deriving the down-sampledneighboring luma reference samples among the neighboring luma referencesamples may be less than that of the left neighboring luma referencesamples used for deriving the down-sampled neighboring luma referencesamples.

The encoding apparatus according to an embodiment may derive a linearmodel parameter based on the down-sampled neighboring luma referencesamples and the neighboring chroma reference samples (S1330). Morespecifically, the prediction module 110 of the encoding apparatus mayderive a linear model parameter based on the down-sampled neighboringluma reference samples and the neighboring chroma reference samples.

The encoding apparatus according to an embodiment may generateprediction samples for the chroma block based on the linear modelparameter and the down-sampled luma samples of the luma block (S1340).More specifically, the prediction module 110 of the encoding apparatusmay generate prediction samples for the chroma block based on the linearmodel parameter and the down-sampled luma samples of the luma block.

The encoding apparatus according to an embodiment may derive residualsamples for the chroma block based on the prediction samples for thechroma block (S1350). More specifically, the residual processing module120 of the encoding apparatus may derive the residual samples for thechroma block based on the prediction samples for the chroma block.

The encoding apparatus according to an embodiment may encode pictureinformation including information about the residual samples (S1360).More specifically, the entropy encoding module 130 of the encodingapparatus may encode picture information including information about theresidual samples.

According to the encoding apparatus disclosed in FIGS. 13 and 14 and anoperation method of the encoding apparatus, when an intra predictionmode of the chroma block is a CCLM mode, the encoding apparatus mayderive neighboring chroma reference samples for the chroma block(S1300), derive neighboring luma reference samples of the luma blockcorresponding to the chroma block and luma samples in the luma block(S1310), down-sample the neighboring luma reference samples and the lumasamples to derive the down-sampled neighboring luma reference samplesand down-sampled luma samples (S1320), derive a linear model parameterbased on the down-sampled neighboring luma reference samples and theneighboring chroma reference samples (S1330), generate predictionsamples for the chroma block based on the linear model parameter and thedown-sampled luma samples of the luma block (S1340), derive residualsamples for the chroma block based on the prediction samples for thechroma block (S1350), and encode picture information includinginformation about the residual samples (S1360), wherein the neighboringluma reference samples include top neighboring luma reference sampleslocated at the upper side of the top boundary of the luma block and leftneighboring luma reference samples located at the left side of the leftboundary of the luma block, and when the top boundary of the luma blockoverlaps with a boundary of a coding tree unit (CTU), the number of thetop neighboring luma reference samples used for deriving thedown-sampled neighboring luma reference samples among the neighboringluma reference samples may be less than that of the left neighboringluma reference samples used for deriving the down-sampled neighboringluma reference samples. That is, when performing CCLM-based intraprediction, by downsampling the neighboring luma reference samples of a1-sample line, image coding efficiency can be improved, and whenimplementing CCLM-based intra prediction in hardware, a pipeline delaycan be improved.

FIG. 15 is a flowchart illustrating an operation of a decoding apparatusaccording to an embodiment, and FIG. 16 is a block diagram illustratinga configuration of a decoding apparatus according to an embodiment.

Each step disclosed in FIG. 15 may be performed by the decodingapparatus 200 disclosed in FIG. 2. More specifically, S1500, S1510, andS1550 may be performed by the adder 240 disclosed in FIG. 2, and S1510to S1540 may be performed by the prediction module 230 disclosed in FIG.2. In addition, operations according to S1500 to S1550 are performedbased on some of the contents described above with reference to FIGS. 3to 12. Therefore, a detailed description repeated with the abovedescription in FIGS. 2 to 12 will be omitted or simply described.

As shown in FIG. 16, the decoding apparatus according to an embodimentmay include a prediction module 230 and an adder 240. However, in somecases, all of the components shown in FIG. 16 may not be essentialcomponents of the decoding apparatus, and the decoding apparatus may beimplemented by more or less components than those illustrated in FIG.16. In one example, the decoding apparatus may further include a memory260.

In the decoding apparatus according to an embodiment, the predictionmodule 230 and the adder 240 may be each implemented into separate chipsor at least two components may be implemented through one chip.

When an intra prediction mode of the chroma block is a CCLM mode, thedecoding apparatus according to an embodiment may derive neighboringchroma reference samples for the chroma block (S1600). Morespecifically, when the intra prediction mode of the chroma block is aCCLM mode, the adder 240 of the decoding apparatus may derive (orreconstruct) neighboring chroma reference samples for the chroma block.

The decoding apparatus according to an embodiment may derive neighboringluma reference samples of the luma block corresponding to the chromablock and luma samples in the luma block (S1610). More specifically, theprediction module 230 and/or the adder 240 of the decoding apparatus mayderive neighboring luma reference samples of the luma blockcorresponding to the chroma block and luma samples in the luma block.

The decoding apparatus according to an embodiment may down-sample theneighboring luma reference samples and the luma samples to derive thedown-sampled neighboring luma reference samples and the down-sampledluma samples (S1620). More specifically, the prediction module 230 ofthe decoding apparatus may down-sample the neighboring luma referencesamples and the luma samples to derive the down-sampled neighboring lumareference samples and the down-sampled luma samples.

In one embodiment, the neighboring luma reference samples include topneighboring luma reference samples located at the upper side of a topboundary of the luma block and left neighboring luma reference sampleslocated at the left side of a left boundary of the luma block, and whenthe top boundary of the luma block overlaps with the boundary of acoding tree unit (CTU), the number of the top neighboring luma referencesamples used for deriving the down-sampled neighboring luma referencesamples among the neighboring luma reference samples may be less thanthat of the left neighboring luma reference samples used for derivingthe down-sampled neighboring luma reference samples.

In an embodiment, when the top boundary of the luma block overlaps withthe boundary of the CTU, the top neighboring luma reference samples maybe included in a horizontal 1-sample line located at the upper side ofthe luma block. Further, the left neighboring luma reference samples maybe included in a plurality of horizontal or vertical sample lines at theleft side of the luma block. For example, the left neighboring lumareference samples may be included in horizontal two-sample lines at theleft side of the luma block or may be included in vertical three-samplelines at the left side of the luma block, but the example is not limitedthereto.

In one embodiment, the down-sampled neighboring luma reference samplesinclude down-sampled left neighboring luma reference samples anddown-sampled top neighboring luma reference samples, and when the topboundary of the luma block overlaps with a boundary of the CTU, thenumber of the top neighboring luma reference samples used for derivingone down-sampled top neighboring luma reference sample may be three, andthe number of the left neighboring luma reference samples used forderiving one down-sampled left neighboring luma reference sample may besix.

In one embodiment, when a coordinate of the down-sampled top neighboringluma reference sample is (x, −1), the three top neighboring lumareference samples may be located at coordinates (2*x−1, −1), (2*x, −1)and (2*x+1, −1), respectively.

In one embodiment, when the down-sampled top neighboring luma referencesample has a coordinate of (0, −1) and there is a sample value of a lumareference sample corresponding to a coordinate (−1, −1), the three topneighboring luma reference samples may be located at coordinates (−1,−1), (0, −1) and (1, −1), respectively.

In one embodiment, when the down-sampled top neighboring luma referencesample has a coordinate of (0, −1) and there is no sample value of theluma reference sample corresponding to the coordinate (−1, −1), in orderto derive the down-sampled top neighboring luma reference sample, onetop neighboring luma reference sample may be used, and a sample value ofthe down-sampled top neighboring luma reference sample may be determinedas a sample value of the top neighboring luma reference sample locatedat a coordinate (0, −1).

In one embodiment, the linear model parameter may include a first linearmodel parameter representing a scaling factor and a second linear modelparameter representing offset.

The decoding apparatus according to an embodiment may derive a linearmodel parameter based on the down-sampled neighboring luma referencesamples and the neighboring chroma reference samples (S1630). Morespecifically, the prediction module 230 of the decoding apparatus mayderive a linear model parameter based on the down-sampled neighboringluma reference samples and the neighboring chroma reference samples.

The decoding apparatus according to an embodiment may generateprediction samples for the chroma block based on the linear modelparameter and the down-sampled luma samples of the luma block (S1640).More specifically, the prediction module 230 of the decoding apparatusmay generate prediction samples for the chroma block based on the linearmodel parameter and the down-sampled luma samples of the luma block.

The decoding apparatus according to an embodiment may reconstruct thechroma block based on the prediction samples for the chroma block(S1650). More specifically, the adder 240 of the decoding apparatus mayreconstruct the chroma block based on the prediction samples for thechroma block.

In one embodiment, in the present specification, CCLM-based intraprediction described above may be implemented, for example, according tothe following spec.

Abstract

This embodiments provides the experimental results of CCLM line bufferrestriction Experimental results from All Intra configuration show0.01%, 0.01%, and 0.04% bit-rate increase compared to VTM2.0.1 on Y, Cb,and Cr components, respectively.

1) Proposed Method

The CCLM (cross-component linear model) method in the current VVCspecification always uses 2 lines of reconstructed luma referencesamples to get the down-sampled collocated luma. In this proposal, toavoid the line buffer increase in the intra prediction, only one lumaline (general line buffer in intra prediction) is used to make thedownsampled luma samples when the upper reference line is at the CTUboundary.

2) Experimental Results

The proposed method has been implemented using BMS-2.0.1 as softwarebase and experimentally evaluated for VTM test according to the commontest conditions defined in JVET-K1010 and Core Experiment description inJVET-K1023. For the VTM based test, the VTM configuration was enabled.

Table 3 and Table 4 show the test results in AI and RA configuration.

TABLE 1 Experimental results of Test1 for all-intra (AI) test condition;anchor is VTM2.0.1 All Intra Main10 Over VTM-2.0.1 Y U V EncT DecT ClassA1 0.00% −0.04% 0.04% 100%  99% Class A2 0.01% 0.06% −0.03% 100% 100%Class B 0.01% 0.03% 0.07% 100%  98% Class C 0.00% 0.01% 0.09% 100% 101%Class E 0.01% −0.04% 0.01% 100% 101% Overall 0.01% 0.01% 0.04% 100% 100%Class D 0.00% 0.00% −0.03% 100%  96%

TABLE 4 Experimental results of Test1 for random-access (RA) testcondition; anchor is VTM2.0.1 Random Access Main 10 Over VTM-2.0.1 Y U VEncT DecT Class A1 −0.03% 0.17% 0.07% 100% 100% Class A2 0.01% −0.08%−0.01%  99%  99% Class B 0.03% 0.04% 0.04% 100% 100% Class C 0.02% 0.16%−0.03% 100% 100% Class E Overall 0.01% 0.08% 0.02% 100% 100% Class D0.02% −0.16% −0.16%  99%  94%

Specification of INTRA_CCLM intra prediction mode with proposed method

Inputs to this process are:

a chroma location (xCbC, yCbC) of the top-left sample of the currentcoding block relative to the top-left sample of the current picture,

a sample location (xTbC, yTbC) of the top-left sample of the currenttransform block relative to the top-left sample of the current picture,

a variable nTbW specifying the transform block width,

a variable nTbH specifying the transform block height,

chroma neighbouring samples p[x][y], with x=−1, y=0 . . . nTbH−1 and x=0. . . nTbW−1, y=−1.

Output of this process are predicted samples predSamples[x][y], with x=0. . . nTbW−1, y=0 . . . nTbH−1.

The current luma location (xTbY, yTbY) is derived as follows:(xTbY,yTbY)=(xTbC<<1,yTbC<<1)

The variables availL, availT and availTL are derived as follows:

The availability of left neighbouring samples derivation process for ablock as specified in clause 6.4.X [Ed. (BB): Neighbouring blocksavailability checking process tbd] is invoked with the current lumalocation (xCurr, yCurr) set equal to (xTbY, yTbY) and the neighbouringluma location (xTbY−1, yTbY) as inputs, and the output is assigned toavailL.

The availability of top neighbouring samples derivation process for ablock as specified in clause 6.4.X [Ed. (BB): Neighbouring blocksavailability checking process tbd] is invoked with the current lumalocation (xCurr, yCurr) set equal to (xTbY, yTbY) and the neighbouringluma location (xTbY, yTbY−1) as inputs, and the output is assigned toavailT.

The availability of top-left neighbouring samples derivation process fora block as specified in clause 6.4.X [Ed. (BB): Neighbouring blocksavailability checking process tbd] is invoked with the current lumalocation (xCurr, yCurr) set equal to (xTbY, yTbY) and the neighbouringluma location (xTbY−1, yTbY−1) as inputs, and the output is assigned toavailTL.

The variable bCTUboudary is derived as follows:bCTUboudary=yCbC&((1<<(Ctb Log 2SizeY−1)−1)==0

The prediction samples predSamples[x][y] with x=0 . . . nTbW−1, y=0 . .. nTbH−1 are derived as follows:

If both availL and availT are equal to FALSE, the following applies:predSamples[x][y]=1<<(BitDepthC−1)

Otherwise, the following ordered steps apply:

1. The collocated luma samples pY[x][y] with x=0 . . . nTbW*2−1, y=0 . .. nTbH*2−1 are set equal to the reconstructed luma samples prior to thedeblocking filter process at the locations (xTbY+x, yTbY+y).

The neighbouring luma samples samples pY[x][y] are derived as follows:

When availL is equal to TRUE, the neighbouring left luma samplespY[x][y] with x=−1 . . . −3, y=0 . . . 2*nTbH−1, are set equal to thereconstructed luma samples prior to the deblocking filter process at thelocations (xTbY+x, yTbY+y).

When availT is equal to TRUE, the neighbouring top luma samples pY[x][y]with x=0 . . . 2*nTbW−1, y=−1, −2, are set equal to the reconstructedluma samples prior to the deblocking filter process at the locations(xTbY+x, yTbY+y).

When availTL is equal to TRUE, the neighbouring top-left luma samplespY[x][y] with x=−1, y=−1, −2, are set equal to the reconstructed lumasamples prior to the deblocking filter process at the locations (xTbY+x,yTbY+y).

3. The down-sampled collocated luma samples pDsY[x][y] with x=0 . . .nTbW−1, y=0 . . . nTbH−1 are derived as follows:

-   -   pDsY[x][y] with x=1 . . . nTbW−1, y=0 . . . nTbH−1 is derived as        follows:

pDsY[x][y] = (pY[2 * x − 1][2 * y] + pY[2 * x − 1][2 * y + 1] + 2 * pY[2 * x][2 * y] + 2 * pY[2 * x][2 * y + 1] + pY[2 * x + 1][2 * y] + pY[2 * x + 1][2 * y + 1] + 4)>> 3

If availL is equal to TRUE, pDsY[0][y] with y=0 . . . nTbH−1 is derivedas follows:

pDsY[0][y] = (pY[−1][2 * y] + pY[−1][2 * y + 1] + 2 * pY[0][2 * y] + 2 * pY[0][2 * y + 1] + pY[1][2 * y] + pY[1][2 * y + 1] + 4)>> 3

Otherwise, pDsY[0][y] with y=0 . . . nTbH−1 is derived as follows:

pDsY[0][y]=(pY[0][2*y]+pY[0][2*y+1]+1)>>1

4. When availL is equal to TRUE, the down-sampled neighbouring left lumasamples pLeftDsY[y] with y=0 . . . nTbH−1 are derived as follows:

pLeftDsY[y] = (pY[−1][2 * y] + pY[−1][2 * y + 1] + 2 * pY[−2][2 * y] + 2 * pY[−2][2 * y + 1] + pY[−3][2 * y] + pY[−3][2 * y + 1] + 4)>> 3

5. When availT is equal to TRUE and bCTUboudary is equal to FALSE, thedown-sampled neighbouring top luma samples pTopDsY[x] with x=0 . . .nTbW−1 are specified as follows:

-   -   pTopDsY[x] with x=1 . . . nTbW−1 is derived as follows:

pTopDsY[x] = (pY[2 * x − 1][−2] + pY[2 * x − 1][−1] + 2 * pY[2 * x][−2] + 2 * pY[2 * x][−1] + pY[2 * x + 1][−2] + pY[2 * x + 1][−1] + 4)>> 3

If availTL is equal to TRUE, pTopDsY[0] is derived as follows:

pTopDsY[0] = (pY[−1][−2] + pY[−1][−1] + 2 * pY[0][−2] + 2 * pY[0][−1] + pY[1][−2] + pY[1][−1] + 4)>> 3

Otherwise, pTopDsY[0] is derived as follows:pTopDsY[0]=(pY[0][−2]+pY[0][−1]+1)>>1

6. When availT is equal to TRUE and bCTUboudary is equal to TRUE, thedown-sampled neighbouring top luma samples pTopDsY[x] with x=0 . . .nTbW−1 are specified as follows:

−pTopDsY[x] with x=1 . . . nTbW−1 is derived as follows:pTopDsY[x]=(pY[2*x−1][−1]+2*pY[2*x][−1]+pY[2*x+1][−1]+2)>>2

If availTL is equal to TRUE, pTopDsY[0] is derived as follows:pTopDsY[0]=(pY[−1][−1]+2*pY[0][−1]+pY[1][−1]+2)>>2

Otherwise, pTopDsY[0] is derived as follows:pTopDsY[0]=pY[0][−1]

7. The variables nS, xS, yS, k0, k1 are derived as follows:nS=((availL&&availT)?Min(nTbW,nTbH):(availL?nTbH:nTbW))xS=1<<(((nTbW>nTbH)&&availL&&availT)?(Log 2(nTbW)−Log 2(nTbH)):0)yS=1<<(((nTbH>nTbW)&&availL&&availT)?(Log 2(nTbH)−Log 2(nTbW)):0)k1=((availL&&availT)? Log 2(nS)+1:Log 2(nS))k0=BitDepthC+k1−15

S. Variables l, c, ll, lc and k1 are derived as follows:l=(availL?Σ _(y=0) ^(nS-1) pLeftDsY[y*yS]:0)+(availT?Σ _(x=0) ^(nS-1)pTopDsY[x*xS]: 0)c=(availL?Σ _(y=0) ^(nS-1) p[−1][y*yS]:0)+(availT?Σ _(x=0) ^(nS-1)p[x*xS][−1]:0)ll=(availL?Σ _(y=0) ^(nS-1)(pLeftDsY[y*yS])²:0)+(availT?Σ _(x=0)^(nS-1)(pTopDsY[x*xS])²: 0)lc=(availL?Σ _(y=0) ^(nS-1) pLeftDsY[y*yS]*p[−1][y*yS]:0)+(availT?Σ_(x=0) ^(nS-1) pTopDsY[x*xS]*p[x*xS][−1]: 0)

9. When k0 is greater than 0, the variable l, c, ll, lc and k1 aremodified as followsl=(l+(l<<(k0−1)))>>k0c=(c+(c<<(k0−1)))>>k0ll=(ll+(ll<<(k0−1)))>>k0lc=(lc+(lc<<(k0−1)))>>k0k1=k1−k0

10. The variables a, b, and k are derived as follows:

If k1 is equal to 0, the following applies:k=0a=0b=1<<(BitDepthC−1)

Otherwise, the following applies:avgY=1>>k1errY=1&((1<<k1)−1)avgC=c>>k1errC=c&((1<<k1)−1a1=lc−((avgY*avgC)<<k1+avgY*errC+avgC*errY)a2=ll−((avgY2)k1+2*avgY*errY)k2=(a1==0)?0:Max(0,Floor(Log 2(Abs(a1)))−BitDepthC+2)k3=(a2==0)?0:Max(0,Floor(Log 2(Abs(a2)))−5)k4=k3−k2+BitDepthC−2a1s=a1>>k2a2s=a2>>k3a2t=(a2s<32)?0:((1<<(BitDepthY+4))+a2s/2)/a2sif (a2s<32)a3=0else if (a2s>=32&&k4>=0)a3=(a1s*a2t)>>k4elsea3=(a1s*a2t)<<(−k4)a4=Clip3(−28,28−1,a3)a5=a4<<7k5=(a5==0)?0:Floor(Log 2(Abs(a5)+(Sign 2(a5)−1)/2))−5k=13−k5a=a5>>k5b=avgC−((a*avgY)>>k)

11. The prediction samples predSamples[x][y] with x=0 . . . nTbW−1, y=0. . . nTbH−1 are derived as follows:predSamples[x][y]=Clip1C(((pDsY[x][y]*a)>>k)+b)

In the specification, it may be determined that in particular, Equations12 to 14 are disclosed.bCTUboundary=yCbC&((1<<(Ctb Log2SizeY−1)−1)==0  [Equation 12]

Concerning Equation 12, whether the top boundary of the luma blockoverlaps with the boundary of the CTU may be determined based on whetherEquation 12 is TRUE or FALSE, and in Equation 12, yCbC may represent ay-axis direction position of an upper left sample of a current chromablock for an upper left sample of a current chroma picture, andCtbLog2SizeY may represent a log value of a luma CTB size.

Next, Equation 13 is as follows.pTopDsY[x]=(pY[2*x−1][−1]+2*pY[2*x][−1]+pY[2*x+1][−1]+2)>>2  [Equation13]

In Equation 13, pTopDsY[x] represents a sample value of a luma referencesample to which downsampling is applied and located at the upper side ofthe top boundary of the luma block, x represents an x-axis position ofthe luma reference sample to which the downsampling is applied,pY[2*x−1][−1], pY[2*x][−1], and pY[2*x+1][−1] represent sample values,respectively of the luma reference samples located at the upper side,and the y-axis position of the luma reference samples located at theupper side is −1.

In an embodiment, when a value of x is 0 and there is the sample valuepY[−1][−1], Equation 13 may be represented with Equation 14.pTopDsY[0]=(pY[−1][−1]+2*pY[0][−1]+pY[1][−1]+2)>>2  [Equation 14]

In Equation 14, pTopDsY[0] represents a sample value of a luma referencesample to which the downsampling is applied and located at the upperside of the top boundary when an x-axis position of the luma referencesample to which the downsampling is applied and located at the upperside of the top boundary is 0, and pY[−1][−1], pY[0][−1], and pY[1][−1]represent sample values of the luma reference samples, respectively,located at the upper side, and a y-axis position of the luma referencesamples located at the upper side is −1.

In an embodiment, when a value of x is 0 and there is no sample value ofpY[−1][−1],

Equation 13 may be represented with Equation 15.pTopDsY[0]=pY[0][−1]  [Equation 15]

In Equation 15, pTopDsY[0] represents a sample value of the lumareference sample to which the downsampling is applied and located at theupper side of the top boundary when an x-axis position of the lumareference sample to which the downsampling is applied and located at theupper side of the top boundary of the luma block is 0, and pY[0][−1]represents a sample value of the luma reference sample located at theupper side of the luma block.

According to a decoding apparatus and a method of operating the decodingapparatus of FIGS. 15 and 16, when an intra prediction mode of thechroma block is a cross-component linear model (CCLM) mode, the decodingapparatus derives neighboring chroma reference samples for the chromablock (S1600), derives neighboring luma reference samples of the lumablock corresponding to the chroma block and luma samples in the lumablock (S1610), down-sample the neighboring luma reference samples andthe luma samples to derive the down-sampled neighboring luma referencesamples and the down-sampled luma samples (S1620), derive a linear modelparameter based on the down-sampled neighboring luma reference samplesand the neighboring chroma reference samples (S1630), generate aprediction sample for the chroma block based on the linear modelparameter and down-sampled luma samples of the luma block (S1640),reconstruct the chroma block based on the prediction samples for thechroma block (S1650), wherein the neighboring luma reference samplesinclude top neighboring luma reference samples located at the upper sideof a top boundary of the luma block and left neighboring luma referencesamples located at the left side of the left boundary of the luma block,and when the top boundary of the luma block overlaps with a boundary ofa coding tree unit (CTU), the number of the top neighboring lumareference samples used for deriving the down-sampled neighboring lumareference samples among the neighboring luma reference samples may beless than that of the left neighboring luma reference samples used forderiving the down-sampled neighboring luma reference samples. That is,when performing CCLM-based intra prediction, by downsampling neighboringluma reference samples of a 1-sample line, image coding efficiency canbe improved, and when implementing CCLM-based intra prediction inhardware, pipeline delay can be improved.

The above-described method according to this document may be implementedin a software form, and the encoding apparatus and/or the decodingapparatus according to this document may be included in an apparatusthat performs image processing of, for example, a TV, a computer, asmartphone, a set-top box, a display device, and the like.

The above-described each part, module, or unit may be a processor or ahardware part that executes successive procedures stored in a memory (orstorage unit). Each step described in the foregoing embodiment may beperformed by a processor or hardware parts. Each module/block/unitdescribed in the foregoing embodiment may operate as hardware/processor.Further, methods suggested by this document may be executed as a code.The code may be written in a processor readable storage medium and maybe thus read by a processor provided by an apparatus.

In the above-described embodiments, methods are described on the basisof a flowchart using a series of steps or blocks, but this document isnot limited to the sequence of steps. Some steps may occursimultaneously or in a different sequence than the steps describedabove. Further, those skilled in the art will understand that the stepsshown in the sequence diagram are not exclusive, that other steps may beincluded, or that one or more steps in the flowchart may be deletedwithout affecting the scope of this document.

When the embodiments of this document are implemented in software, theabove-described method may be implemented by modules (processes,functions, and so on) that perform the functions described above. Suchmodules may be stored in memory and executed by a processor. The memorymay be internal or external to the processor, and the memory may becoupled to the processor using various well known means. The processormay comprise an application-specific integrated circuit (ASIC), otherchipsets, a logic circuit and/or a data processing device. The memorymay include a ROM (read-only memory), a RAM (random access memory), aflash memory, a memory card, a storage medium, and/or other storagedevice.

What is claimed is:
 1. A method for decoding a picture, performed by adecoding apparatus, the method comprising: deriving neighboring chromareference samples for a chroma block; deriving neighboring lumareference samples of a luma block related to the chroma block and lumasamples in the luma block; deriving down-sampled neighboring lumareference samples and down-sampled luma samples by down-sampling theneighboring luma reference samples and the luma samples; deriving alinear model parameter based on the down-sampled neighboring lumareference samples and the neighboring chroma reference samples;generating prediction samples for the chroma block based on the linearmodel parameter and the down-sampled luma samples of the luma block; andreconstructing the chroma block based on the prediction samples for thechroma block, wherein the neighboring luma reference samples includestop neighboring luma reference samples located at the upper side of atop boundary of the luma block, and left neighboring luma referencesamples located at the left side of a left boundary of the luma block,wherein based on the luma block located in a current coding tree unit(CTU) that is different from a CTU in which the top neighboring lumareference samples is located, the number of the top neighboring lumareference samples used for deriving the down-sampled neighboring lumareference samples among the neighboring luma reference samples is lessthan that of the left neighboring luma reference samples used forderiving the down-sampled neighboring luma reference samples, andwherein based on the luma block located in the current CTU that isdifferent from the CTU in which the top neighboring luma referencesamples is located, the top neighboring luma reference samples arelocated in a horizontal 1 sample line adjacent to the top boundary ofthe luma block and the left neighboring luma reference samples arelocated in at least vertical 2 sample lines neighboring the leftboundary of the luma block.
 2. The method of claim 1, wherein thedown-sampled neighboring luma reference samples comprise down-sampledleft neighboring luma reference samples and down-sampled top neighboringluma reference samples, and based on the luma block located in thecurrent CTU that is different from the CTU in which the top neighboringluma reference samples is located, the number of the top neighboringluma reference samples used for deriving one down-sampled topneighboring luma reference sample is three, and the number of the leftneighboring luma reference samples used for deriving one down-sampledleft neighboring luma reference sample is six.
 3. The method of claim 2,wherein the three top neighboring luma reference samples are located atcoordinates (2*x−1, −1), (2*x, −1), and (2*x+1, −1), respectively, whenthe down-sampled top neighboring luma reference sample has a coordinate(x, −1).
 4. The method of claim 2, wherein the three top neighboringluma reference samples are located at coordinates (−1, −1), (0, −1) and(1, −1), respectively, when the down-sampled top neighboring lumareference sample has a coordinate (0, −1) and there is a sample value ofa luma reference sample corresponding to the coordinate (−1, −1).
 5. Themethod of claim 1, wherein the linear model parameter comprises a firstlinear model parameter representing a scaling factor and a second linearmodel parameter representing offset.
 6. A method for encoding a picture,performed by an encoding apparatus, the method comprising: derivingneighboring chroma reference samples for a chroma block; derivingneighboring luma reference samples of a luma block related to the chromablock and luma samples in the luma block; deriving down-sampledneighboring luma reference samples and down-sampled luma samples bydown-sampling the neighboring luma reference samples and the lumasamples; deriving a linear model parameter based on the down-sampledneighboring luma reference samples and the neighboring chroma referencesamples; generating prediction samples for the chroma block based on thelinear model parameter and the down-sampled luma samples of the lumablock; deriving residual samples for the chroma block based on theprediction samples for the chroma block; and encoding pictureinformation including information for the residual samples, wherein theneighboring luma reference samples includes top neighboring lumareference samples located at the upper side of a top boundary of theluma block, and left neighboring luma reference samples located at theleft side of a left boundary of the luma block, wherein based on theluma block located in a current coding tree unit (CTU) that is differentfrom a CTU in which the top neighboring luma reference samples islocated, the number of the top neighboring luma reference samples usedfor deriving the down-sampled neighboring luma reference samples amongthe neighboring luma reference samples is less than that of the leftneighboring luma reference samples used for deriving the down-sampledneighboring luma reference samples, and wherein based on the luma blocklocated in the current CTU that is different from the CTU in which thetop neighboring luma reference samples is located, the top neighboringluma reference samples are located in a horizontal 1 sample lineadjacent to the top boundary of the luma block and the left neighboringluma reference samples are located in at least vertical 2 sample linesneighboring the left boundary of the luma block.
 7. The method of claim6, wherein the down-sampled neighboring luma reference samples comprisedown-sampled left neighboring luma reference samples and down-sampledtop neighboring luma reference samples, and based on the luma blocklocated in the current CTU that is different from the CTU in which thetop neighboring luma reference samples is located, the number of the topneighboring luma reference samples used for deriving one down-sampledtop neighboring luma reference sample is three, and the number of theleft neighboring luma reference samples used for deriving onedown-sampled left neighboring luma reference sample is six.
 8. Themethod of claim 7, wherein the three top neighboring luma referencesamples are located at coordinates (2*x−1, −1), (2*x, −1), and (2*x+1,−1), respectively, when the down-sampled top neighboring luma referencesample has a coordinate (x, −1).
 9. The method of claim 7, wherein thethree top neighboring luma reference samples are located at coordinates(−1, −1), (0, −1) and (1, −1), respectively, when the down-sampled topneighboring luma reference sample has a coordinate (0, −1) and there isa sample value of a luma reference sample corresponding to thecoordinate (−1, −1).
 10. The method of claim 7, wherein, when thedown-sampled top neighboring luma reference sample has a coordinate (0,−1) and there is no sample value of the luma reference samplecorresponding to a coordinate (−1, −1), in order to derive thedown-sampled top neighboring luma reference sample, a one topneighboring luma reference sample is used, and a sample value of thedown-sampled top neighboring luma reference sample is determined as asample value of the top neighboring luma reference sample located at thecoordinate (0, −1).
 11. The method of claim 6, wherein the linear modelparameter comprises a first linear model parameter representing ascaling factor and a second linear model parameter representing offset.12. A non-transitory computer-readable storage medium storing encodedpicture information generated by a method for encoding a picture, themethod comprising: deriving neighboring chroma reference samples for achroma block; deriving neighboring luma reference samples of a lumablock related to the chroma block and luma samples in the luma block;deriving down-sampled neighboring luma reference samples anddown-sampled luma samples by down-sampling the neighboring lumareference samples and the luma samples; deriving a linear modelparameter based on the down-sampled neighboring luma reference samplesand the neighboring chroma reference samples; generating predictionsamples for the chroma block based on the linear model parameter and thedown-sampled luma samples of the luma block; deriving residual samplesfor the chroma block based on the prediction samples for the chromablock; and encoding picture information including information for theresidual samples, wherein the neighboring luma reference samplesincludes top neighboring luma reference samples located at the upperside of a top boundary of the luma block, and left neighboring lumareference samples located at the left side of a left boundary of theluma block, wherein based on the luma block located in a current codingtree unit (CTU) that is different from a CTU in which the topneighboring luma reference samples is located, the number of the topneighboring luma reference samples used for deriving the down-sampledneighboring luma reference samples among the neighboring luma referencesamples is less than that of the left neighboring luma reference samplesused for deriving the down-sampled neighboring luma reference samples,and wherein based on the luma block located in the current CTU that isdifferent from the CTU in which the top neighboring luma referencesamples is located, the top neighboring luma reference samples arelocated in a horizontal 1 sample line adjacent to the top boundary ofthe luma block and the left neighboring luma reference samples arelocated in at least vertical 2 sample lines neighboring the leftboundary of the luma block.
 13. The method of claim 1, wherein based onthe luma block located in the current CTU that is different from the CTUin which the top neighboring luma reference samples is located, the topneighboring luma reference samples used for deriving the down-sampledneighboring luma reference samples are located in the horizontal 1sample line adjacent to the top boundary of the luma block and the leftneighboring luma reference samples used for deriving the down-sampledneighboring luma reference samples are located in vertical 3 samplelines neighboring the left boundary of the luma block.
 14. The method ofclaim 6, wherein based on the luma block located in the current CTU thatis different from the CTU in which the top neighboring luma referencesamples is located, the top neighboring luma reference samples used forderiving the down-sampled neighboring luma reference samples are locatedin the horizontal 1 sample line adjacent to the top boundary of the lumablock and the left neighboring luma reference samples used for derivingthe down-sampled neighboring luma reference samples are located invertical 3 sample lines neighboring the left boundary of the luma block.15. The non-transitory computer-readable storage medium of claim 12,wherein based on the luma block located in the current CTU that isdifferent from the CTU in which the top neighboring luma referencesamples is located, the top neighboring luma reference samples used forderiving the down-sampled neighboring luma reference samples are locatedin the horizontal 1 sample line adjacent to the top boundary of the lumablock and the left neighboring luma reference samples used for derivingthe down-sampled neighboring luma reference samples are located invertical 3 sample lines neighboring the left boundary of the luma block.